This invention relates generally to electromagnetic flowmeters, and more particularly to a noise-compensating arrangement for eliminating from the flow signal noise components arising from an asymmetrical condition in the main magnetic field established in the flowmeter as well as from misalignment of the signal lead wires connected to the flowmeter electrodes.
In a conventional flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube having a pair of diametrically-opposed electrodes mounted thereon. The flowing fluid intercepts a main magnetic field whose lines of flux are perpendicular both to a transverse axis extending across the tube between the electrodes and to the longitudinal axis of the flow tube, as a result of which a voltage is induced in the fluid which is transferred to the electrodes to generate a flow signal that is a function of flow rate.
Ideally, the magnetic field should be perfectly symmetrical with respect to the above-noted transverse axis, but in actual practice this ideal is difficult to attain and the field is somewhat asymmetrical. As a consequence of such asymmetry, a noise voltage is generated between the electrodes which produces an error signal whose largest component is 90.degree. out of phase with the flow signal.
Another source of noise is that produced by misalignment of the signal leads connected to the electrodes. These leads, in combination with the conductive liquid path extending between the electrodes to which the leads are connected, constitute an imaginary loop. Ideally, this loop should lie in a plane which is perpendicular to the excitation coils that produce the magnetic field, but in practice the imaginary loop is disposed in a somewhat inclined plane. As a result of this misalignment, magnetic coupling exists between the excitation coils and the imaginary loop, thereby generating noise which is 90 degrees out of phase with the flow signal. This gives rise to an inaccurate flow rate measurement.